Devices of this type are used for example for reading from and/or writing to compact discs (e.g. CD, CD-R, CD-RW, . . . ) or digital versatile discs (inter alia DVD, DVD-ROM, DVD-RAM, DVD+RW, DVD-RW, . . . ) on which information is present in the form of structures with a deviating reflection behaviour, so-called pits. For this purpose, usually a laser diode emits a linearly polarized light beam which passes through a polarization beam splitter cube and is collimated with the aid of a collimator lens. After the polarization beam splitter cube, the light beam passes through a quarter-wave plate, by which it is circularly polarized, and is focussed onto the recording medium by a converging lens. A part of the light beam is reflected from the recording medium, the intensity of the reflected light beam depending on whether or not the light beam was focussed onto a pit. The reflected light beam is collimated by the converging lens and once again passes through the quarter-wave plate, so that it is linearly polarized again. After passing through the quarter-wave plate twice, the direction of polarization of the reflected light beam is perpendicular to the direction of polarization of the incident light beam, so that the reflected light beam is reflected from the polarization beam splitter cube. Via the polarization beam splitter cube, the light beam is therefore focussed by a collimator lens onto a detector unit which determines the change in the intensity of the reflected light beam and thus generates a data signal which reproduces the sequence of pits on the recording medium. A track error signal and a focus error signal are additionally generated with the aid of the reflected light beam.
The devices according to the prior art have the disadvantage that undesirable modulations can occur in the said signals if the recording medium itself influences the polarization of the light beam, but this influence varies in magnitude for different regions of the recording medium. Such an influence on the polarization is caused, for example, by birefringence, which is brought about by the orientation of the molecules, strains or inclusions in the material of the recording medium, but also by the reflection of the light beam at the information-carrying metal layer. The reflected light beam is elliptically polarized by the birefringence. Since only that component of the reflected light beam whose polarization is perpendicular to the polarization of the incident light beam is reflected from the polarization beam splitter cube, the influence on the polarization caused by the recording medium leads to a modulation of the intensity of the light beam incident on the detector unit. This can lead to problems when reading out the stored data if the modulations exceed predetermined tolerances.
Therefore, it is an object of the invention to propose a device for reading from and/or writing to optical recording media which avoids the abovementioned problems and ensures correct reproduction of the stored data.